Astropitches have a tendency to react to moisture and crease. I have found that on the original Subbuteo ones an iron tends to make them worse. It will be difficult to remove the creases unless you can (semi) permanently lay the pitch on a table top or board. You could try attaching the pitch with double-sided sticky tape BUT ONLY in the four corners, then really stretching-pulling on each corner to remove the creases. I have used this method and it works well, for a few months, but overtime the creases might return, obviously it's quite easy to redo. Alternatively stick the pitch down permanently but again before the glue dries stretch-pull the four corners and then put heavy books on them to hold in place.